1. Field of the Invention
The present invention relates to a communication method suitable for application to a base station and a terminal apparatus of a radio telephone system, for example, and a base station and a terminal apparatus to which the communication method is applied.
2. Description of the Related Art
In a mobile communication system such as a radio telephone system or the like, multiple access where a plurality of mobile stations (terminal apparatus or subscribers) are permitted to access a single base station is employed. In the case of a radio telephone, a number of mobile stations commonly utilize a single base station. Therefore, various communication systems have been proposed for avoiding interference between respective mobile stations. For example, a frequency division multiple access (FDMA) system, a time division multiple access (TDMA) system, a code division multiple access (CDMA) system and so on are conventionally proposed as this kind of communication systems.
Of these systems, the CDMA system is a multiple access system in which a particular code is assigned to each of the mobile stations, a modulated wave of an identical carrier wave (carrier) is spread in spectrum with the code and then transmitted to the identical base station, and a base station receives it with taking code synchronism based on each code to identify a desired mobile station.
Specifically, the base station occupies the whole frequency band owing to the spectrum, and transmits signals to a plurality of mobile stations using an identical frequency band at the same time. Each of the mobile stations inversely spreads a signal of a fixed spread band width transmitted from the base station to extract a corresponding signal. Further, the base station discriminates each of the mobile stations by different spread codes from one another.
In the CDMA system, communication can be achieved at every direct calling so long as a code is shared. Further, the system is excellent in secrecy of telephone conversation. Therefore, the system is suitable for a radio transmission utilizing mobile stations such as a portable telephone apparatus and so on.
The assignee of the present application has proposed a communication system called a band division multiple access (BDMA) system (disclosed in Japanese patent application No. 132434/1996 and so on) as another communication system. While details of the BDMA system will be described in the detailed description of the preferred embodiment later on, the BDMA system is as follows in short. A plurality of transmission bands in each of which subcarrier signals of a predetermined number are disposed at a predetermined frequency interval are prepared. A signal in each of the transmission bands is divided by every predetermined time to form time slots. A burst signal is transmitted in the form of a multicarrier signal modulated by dispersing the signal intermittently into the subcarrier signals of the above predetermined number at a period of the time slots of a predetermined number. This BDMA system has excellent transmission characteristics.
A system having a frequency allocation called one-cell repetition system has been proposed as a radio telephone system to which the above communication systems is applied. In this one-cell repetition system, a frequency band used in a cell formed of each of base stations is made common in all the cells. In this case, since a frequency used in each of the cells is the same, there can be achieved an effect in which a system arrangement used when the base stations are located to form cells is considerably simplified.
However, if the one-cell repetition system is employed, then the same frequency is a used even in adjacent cells. Therefore, since there is high possibility that communication at one station will interfere with cells adjacent to the station, the one-cell repetition system can only be applied to a communication system having excellent selectivity of a specific path such as the above CDMA system or the above BDMA system.
Moreover, even if the above CDMA system or the above BDMA system having excellent selectivity of a specific path is employed, it is frequently observed that communication in one station interferes with cells adjacent thereto depending upon the communication state.